Add clang format check to one of the builds to provide indication that formatting is incorrect

.travis.yml

@@ -18,8 +18,9 @@ matrix:
     - os: linux
       compiler: clang
       sudo : true
+      env: TEST_CLANG_FORMAT="yes"
       install: ./ci/install-linux.sh && ./ci/log-config.sh
-      script: ./ci/build-linux-bazel.sh
+      script: ./ci/test_format.sh && ./ci/build-linux-bazel.sh
     - os: linux
       group: deprecated-2017Q4
       compiler: gcc
@@ -65,6 +66,7 @@ addons:
     packages:
     - g++-4.9
     - clang-3.9
+    - clang-format-3.9
 
 notifications:
   email: false